(1) Field of the Invention
The present invention relates to a molecularly oriented, silane-crosslinked ultra-high-molecular-weight polyethylene molded article and a process for the preparation thereof. More particularly, the present invention relates to a molded article which has high elastic modulus and high tensile strength inherent to a draw-molded article of ultra-high-molecular-weight polyethylene and which is prominently improved in heat resistance and adhesiveness, and a process for the preparation thereof.
(2) Description of the Prior Art
It is known that a molecularly oriented molded article having high elastic modulus and high tensile strength can be obtained by shaping ultra-high-molecular-weight polyethylene into a fiber, a tape or the like and drawing the shaped article. For example, Japanese Patent Application Laid-Open Specification No. 15408/81 teaches that a dilute solution of ultra-high-molecular-weight polyethylene is spun and the obtained filament is drawn. Furthermore, Japanese Patent Application Laid-Open Specification No. 130313/84 teaches that ultra-high-molecular-weight polyethylene is melt-kneaded with a wax, the kneaded mixture is extruded, cooled and solidified and the solidified extrudate is drawn. Moreover, in Japanese Patent Laid-Open Specification No. 187614/84, it is taught that a melt-kneaded mixture as described above is extruded and the extrudate is drafted, cooled and solidified, and then drawn.
Separately, it is known that a polyolefin is crosslinked with a silane to improve a heat resistance and the like to the polyolefin. For example, Japanese Patent Publication No. 1711/73 teaches a process in which a silane compound is grafted to polyethylene in the presence of a radical generator and the grafted polyethylene is crosslinked by exposing it to water in the presence of a silance condensation catalyst. Furthermore, Japanese Patent Application Laid-Open Specification No. 11154/79 teaches that a silane-grafted polyolefin molded article is dipped in a liquid mixture of a silanol condensation catalyst and a solvent to expedite the crosslinking treatment, and Japanese Patent Application Laid-Open Specification No. 154872/77 teaches that a molecularly oriented, silane-grafted polyolefin is cross-linked and is then subjected to an extraction treatment.
A draw-molded article of ultra-high-molecular-weight polyethylene, for example, a fiber or tape, has high elastic modulus and high tensile strength, is light in the weight and is excellent in water resistance and weatherability, but it still has inherent defects of polyethylene, that is, poor heat resistance and poor adhesiveness.
Moreover, according to the conventional technique of crosslinking polyethylene with a silane, a draw-molded article having high elastic modulus and high tensile strength cannot be obtained, and the effect of improving the heat resistance is insufficient.
It is known that if polyethylene is molecularly oriented or crosslinked, the heat resistance of polyethylene is generally improved. However, according to this conventional technique, the improvement of the heat resistance is limited, and the inherent defect of polyethylene, that is, the relatively low melting point of 110.degree. to 140.degree. C., cannot be overcome, and so far as we know, most of molded articles of polyethylene are fused if they are exposed to a temperature of 180.degree. C. for 10 minutes, and the strength is lost.